linux-kernel - Re: [patch 1/4] Add routine for generating an ID for kernel pointer

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Date:	Tue, 27 Dec 2011 15:23:44 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
Cc:	linux-kernel@...r.kernel.org,
	Pavel Emelyanov <xemul@...allels.com>,
	Glauber Costa <glommer@...allels.com>,
	Andi Kleen <andi@...stfloor.org>, Tejun Heo <tj@...nel.org>,
	Matt Helsley <matthltc@...ibm.com>,
	Pekka Enberg <penberg@...nel.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	Alexey Dobriyan <adobriyan@...il.com>
Subject: Re: [patch 1/4] Add routine for generating an ID for kernel pointer

On Fri, 23 Dec 2011 16:47:42 +0400
Cyrill Gorcunov <gorcunov@...nvz.org> wrote:

> The routine XORs the given pointer with a random value
> producing an ID (32 or 64 bit, depending on the arch).
> 
> Since it's a valuable information -- only CAP_SYS_ADMIN
> is allowed to obtain it.
> 
>  - Tejun worried about the single poison value was a weak side -
>    leaking one makes all the IDs vulnerable. To address this
>    several poison values - one per object type - are introduced.
>    They are stored in a plain array.
>  - Pekka proposed to initialized poison values in the late_initcall callback
>  - ... and move the code to mm/util.c

I'm trying to remember what this is all about, and I don't want to have
to remember all the discussion from last time this came up!

Please, do cover all this in the changelogs: tell us what the code is
all for and try to capture the design decisions thus far.  It's a
useful reminder for current reviewers and is very valuable for new
reviewers.

The root-only restriction sounds like a pretty bad one.  I suspect it
really isn't that bad but again, the changelog should discuss the pros
and cons here.

A thought: if all we're trying to do here is to check for the sameness
of objects, can we push the comparison into the kernel so we don't have
this exporting-sensitive-info problem at all?  Just return a boolean to
userspace?

Something like

int sys_pid_fields_equal(pid_t pid1, pid_t pid2, enum pid_field field_id);

?

For /proc/pid/fdinfo/* userspace can open /proc/pid1/fdinfo/0 and
/proc/pid2/fdinfo/0 and call sys_are_these_files_the_same(fd1, fd2, ...).

Perhaps sys_pid_fields_equal() can use sys_are_these_files_the_same()
as well, if we can think up a way of passing it two fds to represent
the two pids.

Have a think about it ;)
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